Ink jet printers have become a very common way for printing images from a computer. Ink jet printers work by spraying small drops of colorants (ink) onto a receiver to form an image. Typically, ink jet printers use four or more different colors of colorants to produce colored images. Most commonly cyan (C), magenta (M), yellow (Y), and black (K) colorants are used. Different types of ink having different chemical compositions are known in the art. Two common types of ink are dye-based inks and pigment-based inks. Each of these ink types are known to have certain advantages and disadvantages. Dye-based inks are known to produce a wide range of colors, but have poor image durability characteristics, and are subject to fading or damage over time with exposure to light or moisture. The term “gloss” refers to light, which is reflected off of the front surface of the print, and appears when an image is viewed in a near specular orientation. Pigmented inks are known to provide good image durability characteristics, but can suffer from gloss artifacts (any unexpected appearance of gloss) that result in a perceived image quality loss. These gloss artifacts include “differential gloss”, which is an abrupt undesirable change in gloss appearing between two adjacent regions in an image; “chromatic gloss”, which is an undesirable change in the color of the gloss that appears when an image is viewed in a near specular orientation; and “haze”, which refers to a cloudy or smoky appearance to an image resulting from light scattering off of the surface of the print.
Several methods to address the undesirable gloss artifacts described above are known in the art. One technique known in the art is to laminate the print, but this is typically too time-consuming and costly. Another technique is to apply an additional, substantially clear ink to the entire image during or shortly after the printing process. For example, see U.S. Pat. Nos. 6,428,157, and 6,561,644. The application of a full layer of clear ink on top of an area printed with pigmented inks is likely unnecessary to achieve the desired mitigation of gloss artifacts, and is wasteful of ink. Also, indiscriminate application of clear ink leads to a dramatic increase in the total amount of fluid deposited on the page, which is known to cause other negative image quality artifacts. See for example U.S. Pat. No. 6,435,657.
Other techniques known in the art attempt to reduce differential gloss by applying a clear ink in unprinted areas. See for example U.S. Pat. Nos. 6,857,733, 6,953,244, and 6,863,392.
In U.S. patent application Ser. No. 6,877,850, a method of applying clear ink based on the total duty of the colored ink is disclosed. Similarly, U.S. Pat. No. 6,585,363 to Tanaka, et al., discloses a method of applying a clear ink in which the CMYK ink amounts are summed to generate a map of printed pixels. The map is then “thinned” using a masking process to determine which locations will receive the clear ink.
The above mentioned references teach the use of a clear ink for improving some of the aforementioned gloss artifacts, but do not teach methods of controlling the laydown of the clear ink in response to the mixture of colored ink that will be printed. For example, the gloss properties of the different colored inks can be different, thereby requiring different amounts of clear ink to be applied to reduce differential gloss based on the mixture of the colored inks that are printed. Thus, there is a need for a method of computing a clear ink amount to be applied to an image to provide for improved image quality by minimizing gloss related artifacts, while minimizing the total amount of fluid deposited on the page by not printing clear ink where it is unnecessary.